This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to 9923378.5 filed in United Kingdom on Oct. 5, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to signalling over the Gs interface in a GSM mobile telecommunications network and more particularly to the BSSAP+ protocol used to communicate over this interface.
The Global System for Mobile communications (GSM) is a widely used mobile telecommunications system defined by the European Telecommunications Standards Institute (ETSI). However, the current GSM system has only a limited capacity to transmit data because it makes use of circuit switched channels. Circuit switched channels occupy large amounts of bandwidth and produce a bottleneck for data transmission.
ETSI has therefore specified an enhancement to GSM (GSM Phase 2) and which is known as the GSM Packet Radio Services (GPRS). GPRS is a packet switched technology which transmits user data in the form of packets. Packets are only sent when there is data to send, i.e. it is no longer necessary to allocate an entire channel to a single user and data channels may be shared between many users. This frees the data transmission bottleneck of GSM. FIG. 1 illustrates schematically a GSM Phase II network which includes a GPRS packet switched network 1 as well as a conventional GSM circuit switched network 2. The GPRS backbone 3 is coupled for example to the Internet 4 via a Gateway GPRS Support Node (GGSN) 5.
In the GPRS network, the gateway 6 between the GPRS backbone 3 and a Base Station Controller (BSC) 7 which is responsible for a given mobile subscriber 8 via a Base Transceiver Station (BTS) 9, is known as a Serving GPRS Support Node (SGSN). A SGSN 6 is in some ways analogous to a Mobile Switching Centre (MSC) 10 of the GSM network 2, in that a SGSN 6 deals with the registration of mobile subscribers as well as the initial routing of subscriber connections. As with the MSC 10, for roaming subscribers a SGSN 6 must in certain cases exchange information with a Visitor Location Register (VLR) 11 located in the GSM network 2 (the GSM network also having a Home Location Register (HLR) 12). The interface between a SGSN 6 and the VLR 11 (over an SS7 network 13xe2x80x94see below) is identified as the Gs interface. For this purpose ETSI has defined a protocol known as the Base Station System Application Protocol+(BSSAP+) and which is specified in ETSI recommendation""s GSM 09.16 and GSM 09.18.
Signalling messages are typically transported between signalling points of a telecommunications system using a Signalling System No.7 (SS7) network 13. The protocol stack for SS7 is illustrated in FIG. 2 and comprises three Message Transfer Parts (MTP levels 1 to 3) and a Signalling Connections and Control Part (SCCP). The nature of the MTP layers will not be described in detail here other than to note that they perform routing and error correction roles amongst other things.
SS7 is used by a number of application and user parts such as an ISDN User Part (ISUP), a Telephony User Part (TUP), a Transaction Capabilities Application Part (TCAP), and the BSSAP+ protocol. BSSAP+ (like TCAP) makes use of SS7 via the SCCP. A role of the SCCP is to provide subsystem numbers to allow messages to be addressed to specific applications (called subsystems) at network signalling points.
FIG. 3 illustrates a typical sequence of signalling messages over the Gs interface, between a SGSN and a VLR using the BSSAP+ protocol. This sequence arises during a location update, e.g. when a mobile subscriber enters the coverage area of a new BSC, and comprises: a BSSAP+ Location Update Request sent from the SGSN to the VLR; a BSSAP+ Location Update Accept returned from the VLR to the SGSN; and a BSSAP+ TMSI reallocation complete message sent from the SGSN to the VLR.
The inventor of the present invention has recognised that problems may arise when a mobile subscriber makes a new location update request whilst the system is still processing an earlier request. This situation may arise for example when the subscriber turns the power off on his mobile terminal whilst the system is processing a location update request, and is illustrated in FIG. 4.
Each GSM mobile subscriber is allocated an International Mobile Subscriber Identity (IMSI). This code is sent to the network from the subscriber""s terminal during the registration process and is included in the BSSAP+ Location Update Request sent from the SGSN to the VLR. The IMSI is the only identity for making associations in the Gs interface. As such, neither the VLR nor the SGSN are able to know for certain to which transaction a message belongs. Current GSM specifications require the VLR to terminate an existing transaction in the event that another BSSAP+ Location Update Request is received containing the same IMSI as is present in the pre-existing transaction. As the delivery of messages takes a finite time, the SGSN is not able to know whether a received Location Update Accept message corresponds to the first or second Location Update Request message which the SGSN sent to the VLR. This is unfortunate, as the SGSN must act in different ways depending upon whether the Location Update Accept message relates to the first or the second Location Update Request. If the Location Update Accept message relates to the first Location Update Request, the message should be ignored by the SGSN, and the SGSN should wait for the second Location Update Accept message, otherwise it should proceed to act upon the Location Update Accept message. A consequence of the SGSN acting mistakenly on the first Location Update Accept message is that the SGSN may send an incorrect Temporary Mobile Subscriber Identity (TMSI) to a subscriber, resulting in the subscriber not being able to receive calls when the network is paging the subscriber with another TMSI.
It is an object of the present invention to overcome or at least mitigate the above noted disadvantages. It is a second object of the present invention to enable messages sent over a Gs interface to be identified with a particular transaction. These and other objects are achieved at least in part by incorporating a transaction identifier into BSSAP+ protocol messages or by appending such an identifier to these messages.
According to a first aspect of the present invention there is provided a method of sending BSSAP+ messages over the Gs interface between a Serving GPRS Support Node (SGSN) and a Visitor Location Register (VLR) of a GSM mobile telecommunications system, the method comprising incorporating into BSSAP+ messages, or appending thereto, an identifier which allows the messages to be associated with the transactions to which they relate.
As a Location Update Accept message will contain or be accompanied by the same identifier as the Location Update Request message to which it relates, the participating SGSN can unequivocally associate the two messages when the present invention is employed.
In certain embodiments of the present invention BSSAP+ messages may be encapsulated within TCAP messages prior to further encapsulation within SCCP messages. In this case, said identifier is a sequence number (transaction ID) which is included in the TCAP message header at the signalling point which initiates the transaction.
In other embodiments of the present invention, the BSSAP+ protocol message structure is modified to allow for the inclusion of a sequence number into BSSAP+ messages. A signalling point receiving a message includes the received sequence number in any response message.
According to a second aspect of the present invention there is provided a Serving GPRS Support Node (SGSN) or a Visitor Location Register (VLR) for use in a GSM mobile telecommunications network, the SGSN or VLR comprising:
processing means arranged in use to incorporate into BSSAP+ messages, or append thereto, an identifier which allows the messages to be associated with the transactions to which they relate; and
transmission means for causing said BSSAP+ messages, incorporating said identifier or having said identifier appended thereto, to be transmitted to a peer SGSN or VLR over an intervening Gs interface.
In certain embodiments of the above second aspect of the invention, said processing means is arranged to implement the TCAP protocol wherein sequence numbers are included in a header of the TCAP messages.
In other embodiments of the above second aspect of the invention, said processing means is arranged to implement the BSSAP+ protocol wherein sequence numbers are incorporated into the BSSAP+ messages.